An internal combustion engine combusts an air/fuel mixture within cylinders to produce torque, which may be used to propel a vehicle. During vehicle operation, an engine controller regulates the amount of torque produced by the engine by controlling various operating parameters, including amounts of air and fuel provided to the cylinders and a timing of a spark produced by a spark plug to initiate combustion. A torque reduction may be requested due to a gear shift event or responsive to a driver tip-out event, among other torque reduction conditions. The controller may retard spark timing, throttle airflow, and/or cut fuel in response to the torque reduction request. Frequently, retarded spark timing is used to quickly reduce the amount of torque produced by the engine while maintaining the amount of fuel and air provided to the cylinders, whereas throttling airflow may be used when a slower response is sufficient.
However, the further spark timing is retarded from MBT timing, the lower the combustion stability and the greater the likelihood that misfire may occur. When a misfire occurs, no torque is produced by the firing event. Based on engine speed, there may be a spark timing range where the likelihood of random misfire occurrence increases. If spark is retarded into that region (herein also referred to as an unstable spark region) to provide the requested torque reduction, due to random occurrences of misfire, the amount of engine torque actually produced may significantly differ from what is requested. Further, since the amount of engine torque produced is unpredictable due to the randomness of the misfire occurrence, it may be difficult to compensate for.
One example approach for reducing random misfires while using spark retard to reduce engine torque is shown by Gwidt et al. in U.S. Pat. No. 8,332,127 B2. Therein, if a single injection of fuel is not predicted to combust at the retarded spark timing (e.g., due to misfire occurrence), a second fuel injection is provided. The second injection serves to form a rich cloud of fuel near the spark plug, which facilitates combustion when the retarded spark is provided. The net spark timing using the two fuel injections may be further retarded from the spark timing of the original single injection, and torque reduction is provided without misfire occurring.
However, the inventors herein have recognized potential issues with such systems. As one example, the actual torque reduction provided may be higher than desired. In other words, the reduction in misfire occurrence is associated with a torque penalty. In still other approaches, to operate spark timing outside of the unstable region, an engine controller may round the torque reduction request up or down. However, in all such cases, the actual spark retard provided is more or less than the spark retard that was requested. The resulting torque errors may cause engine performance to be degraded. If the torque reduction was requested during a transmission gear shift, the excess or deficient torque reduction may make the shift perceptible, and objectionable, to the operator. In addition, engine response during each torque reduction event may vary, reducing vehicle drivability. Further, the random misfire occurrence may cause the misfire count on a misfire monitor to be incremented, triggering misfire mitigation actions that may further disrupt the requested torque reduction.
In one example, the issues described above may be addressed by a method for an engine comprising: responsive to an estimated spark timing for a requested torque reduction being between an upper threshold and a lower threshold, adjusting a spark timing for each firing event of a plurality of firing events to bring an average spark timing over the plurality of firing events to the estimated spark timing. In this way, a requested torque reduction can be provided while reducing random misfire occurrence.
As one example, responsive to a torque reduction request, such as due to a transmission gear upshift, an engine controller may determine an amount of spark timing retard to apply to each cylinder over an engine cycle to provide the requested torque reduction. If the target retarded spark timing falls in an unstable region where random misfires can occur, the controller may recalculate the spark timing for each cylinder so that the average spark timing across the cylinders is maintained at the target spark timing. For example, the spark timing retard applied for a first cylinder (or first number of cylinders) may be adjusted to be lower than the target spark timing retard, resulting in less torque reduction for that cylinder(s). The spark timing retard applied for a second cylinder (or second number of cylinders) may be adjusted to be higher than the target spark timing retard, resulting in more torque reduction for that cylinder(s). By adjusting the number and identity of cylinders that get more or less torque than desired, the average spark timing can be adjusted to provide the requested torque reduction. For example, the unstable region may be defined by upper and lower thresholds, and based on the position of the target retarded spark timing within the unstable region, relative to the upper and lower thresholds, the spark timing of individual cylinders may be adjusted. This may include adjusting spark timing over the cylinders symmetrically (e.g., in a four cylinder engine, two cylinders may be moved above the upper threshold while another two are moved below the lower threshold) or asymmetrically (e.g., in a four cylinder engine, one cylinder may be moved above the upper threshold while the remaining three are moved below the lower threshold). As such, cylinders whose spark timings are moved below the lower threshold may misfire; however, since this is a planned and controlled misfire, a misfire count is not incremented and a misfire monitor is not triggered.
In this way, a desired amount of torque reduction can be provided while operating an engine with spark timing outside of an unstable region. The technical effect of adjusting spark timing for some cylinders to provide more torque reduction than desired while adjusting the spark timing of other cylinders to provide less torque reduction than desired is that any amount of torque reduction can be provided, on average, through the use of spark retard. By reducing the need to round up or down a spark timing to reduce random misfire occurrence, engine torque errors are reduced, while allowing for a more consistent engine response and a smoother shift between gears. By shifting the spark timing of some cylinders to a region where misfire occurrence can be controlled, the drop in torque provided by the controlled misfire event can be advantageously used to provide the requested torque reduction. Overall, engine performance is improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.